Shedding Light on CO Oxidation Surface Chemistry on Single Pt Catalyst Nanoparticles Inside a Nanofluidic Model Pore

Abstract

Investigating a catalyst under relevant application conditions is experimentally challenging and parameters like reaction conditions in terms of temperature, pressure, and reactant mixing ratios, as well as catalyst design, may significantly impact the obtained experimental results. For Pt catalysts widely used for the oxidation of carbon monoxide, there is keen debate on the oxidation state of the surface at high temperatures and at/above atmospheric pressure, as well as on the most active surface state under these conditions. Here, we employ a nanoreactor in combination with single-particle plasmonic nanospectroscopy to investigate individual Pt catalyst nanoparticles localized inside a nanofluidic model pore during carbon monoxide oxidation at 2 bar in the 450–550 K temperature range. As a main finding, we demonstrate that our single-particle measurements effectively resolve a kinetic phase transition during the reaction and that each individual particle has a unique response. Based on spatially resolved measurements, we furthermore observe how reactant concentration gradients formed due to conversion inside the model pore give rise to position-dependent kinetic phase transitions of the individual particles. Finally, employing extensive electrodynamics simulations, we unravel the surface chemistry of the individual Pt nanoparticles as a function of reactant composition and find strongly temperature-dependent Pt-oxide formation and oxygen spillover to the SiO 2 support as the main processes. These results therefore support the existence of a Pt surface oxide in the regime of high catalyst activity and demonstrate the possibility to use plasmonic nanospectroscopy in combination with nanofluidics as a tool for in situ studies of individual catalyst particles.

Thermodynamic Equilibrium versus Kinetic Trapping: Thermalization of Cluster Catalyst Ensembles Can Extend Beyond Reaction Time Scales

“…9 Furthermore, different isomers can have distinct reactivities where metastable isomers have the potential to be more reactive than the most abundant stable structure. 3,10−13 The importance of structures to reactivity has also been seen for single-nanoparticle catalysts, 14 indicating the importance of taking into account the full structural diversity when attempting to identify structure−property relationships. 15,16 Naturally, determining the exact structure on the atomistic level is difficult.…”

“…These sub-nanoclusters have also been shown to be highly fluxional, meaning that they can populate and interconvert between multiple isomers at catalytically relevant temperatures , and even dramatically change morphology within the ensemble as the partial pressure of reactants changes . Furthermore, different isomers can have distinct reactivities where metastable isomers have the potential to be more reactive than the most abundant stable structure. ,− The importance of structures to reactivity has also been seen for single-nanoparticle catalysts, indicating the importance of taking into account the full structural diversity when attempting to identify structure–property relationships. , Naturally, determining the exact structure on the atomistic level is difficult . Additionally, catalyst dynamics are an important part of the reaction coordinate; dynamic restructuring has been observed experimentally, − and multiple DFT studies have supported the fact that restructuring of active sites is relevant for many catalytic reactions. − Furthermore, Imaoka et al have explicitly shown the isomerization of a Pt 4 cluster in real time using HAADF-STEM, visualizing the exact kinds of structural change addressed in this paper.…”

Mechanistic Insights Gained by High Spatial Resolution Reactivity Mapping of Homogeneous and Heterogeneous (Electro)Catalysts

“…35 These measurements provided kinetic analysis of phase transition mechanism in single metallic NPs under working conditions. 36 The main restriction of the methods presented thus far is their limited ability to acquire chemical information at the nanoscale that will uncover the nature and distribution of reactants, intermediates, and products. 37−39 Nanoscale chemical information under working conditions is vital for determining the influence of different sites on the observed catalytic performances of heterogeneous and homogeneous (electro)catalysts.…”

“…The combination of single-particle plasmonic nanospectroscopy and mass-spectrometry was utilized to correlate single-particle and ensemble-based reactivity . These measurements provided kinetic analysis of phase transition mechanism in single metallic NPs under working conditions …”

Label-Free Imaging of Catalytic H2O2 Decomposition on Single Colloidal Pt Nanoparticles Using Nanofluidic Scattering Microscopy

“…To contribute to the quest of experimental method development that enables efficient and quantitative scrutiny of catalytic reactions on single nanoparticles, we have recently introduced the concept of nanofluidic reactors and used them in combination with plasmonic imaging and spectroscopy, together with mass spectrometry in the gas phase − and with fluorescence microscopy in the liquid phase, using both nanofabricated particles and individually trapped colloidal nanocrystals as the catalyst. , One of the key traits of this nanofluidic reactor platform is that it ensures identical reaction conditions for the individual particles during an experiment while isolating each of them in its own nanochannel. Thereby, it enables highly parallelized studies of tens of single nanoparticles in the same experiment, whereby it eliminates errors and uncertainties inherent to the subsequent experiments traditionally used.…”